Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic type of metal deposition that involves several simultaneous reactions in an aqueous solution, which may occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, such as sodium borohydride, and is oxidized thus producing a negative charge on the surface of the part. The most common electroless plating metals are nickel and copper, where the chemistry for each may vary.
The chemical reaction following the deposition of electroless copper indicates that approximately 22.4 liters of hydrogen may be released for every 63.5 grams of copper deposited. According to one estimate, about 350 cc of hydrogen is released for the deposition of 1 gm of electroless copper. The following equation further elaborates the chemical reaction:Cu+2+2HCHO+4OH−→Cu0+2HCOO−+H2  (1)
The hydrogen release creates porosity in the copper. If further copper is added on top of a layer produced by electroless copper deposition, for example, via electroplating, the added copper has a tendency to retain porosity from the lower layer. From a practical manufacturing perspective, the porosity appears in the final conductive lines and affects circuitization, especially very fine line circuitry. The porosity reduces adhesion of the added copper as the metal layers may easily peel off, and is detrimental to electrical performance of the final circuit.
The hydrogen released due to the chemical reactions, occurring during the deposition of electroless copper, may also be absorbed by substrates such as polyimid, polyester, epoxy boards, and any other substrates. The absorbed hydrogen is very slowly released from the substrate and the trapped hydrogen may influence the mechanical and electrical properties of the deposited copper layer.